Estate tea simply means it comes from one estate or farm, and is most often used when describing Darjeeling. Glenburn Estate is the the Glenburn brand grown on their farm. It also indicates that it is one crop and not blended. So you are getting a quality leaf which has certain characteristics of that farm and you can buy from year to year knowing, at least somewhat, what to expect.

Of course many oolongs are processed the same way, from single "estates" and could easily call themselves as such. But it is more of an Indian terminology thing.